IK Multimedia SampleTank, £13.99

If you want to turn your iOS device into a sound module for performance use, SampleTank is the solution.

136 instruments come as standard (though four are only available once you register) and this library can be expanded to more than 500 via in-app purchases. There are 16 instrument categories to choose from, plus more than 1000 patterns for groove creation. A 4-track MIDI recorder enables you to record you own parts.

SampleTank sounds more than acceptable and, when used in conjunction with a MIDI keyboard, can become very useful.